Method and apparatus for controlling a combined heat and power fuel cell system

ABSTRACT

A cogeneration fuel cell system and associated methods of operation are provided that accommodate a demand for heat as well as a demand for electric power. The system is operated among various modes to balance heat and power demand signals. In general, a fuel cell system is coupled to a power sink and a heat sink, and a controller is adapted to respond to data signals from the power sink and the heat sink. As examples, such data signals from the heat sink may include a temperature indication or a heat demand signal (such as from a thermostat), and such data signals from the power sink may include a voltage or current measurement, an electrical power demand signal, or an electrical load.

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority under 35 USC 119(e) from U.S.Provisional Application No. 60/294,776, filed May 31, 2001, namingBallantine, Hallum, Parks and Skidmore as inventors, and titled “METHODAND APPARATUS FOR CONTROLLING A COMBINED HEAT AND POWER FUEL CELLSYSTEM” That application is incorporated herein by reference in itsentirety and for all purposes.

BACKGROUND

[0002] The invention generally relates to a combined heat and power fuelcell system and associated methods of operation.

[0003] A fuel cell is an electrochemical device that converts chemicalenergy produced by a reaction directly into electrical energy. Forexample, one type of fuel cell includes a polymer electrolyte membrane(PEM), often called a proton exchange membrane, that permits onlyprotons to pass between an anode and a cathode of the fuel cell. At theanode, diatomic hydrogen (a fuel) is reacted to produce protons thatpass through the PEM. The electrons produced by this reaction travelthrough circuitry that is external to the fuel cell to form anelectrical current. At the cathode, oxygen is reduced and reacts withthe protons to form water. The anodic and cathodic reactions aredescribed by the following equations:

H₂→2H⁺+2e⁻

[0004] at the anode of the cell, and

O₂+4H⁺+4e⁻→2H₂O

[0005] at the cathode of the cell.

[0006] A typical fuel cell has a terminal voltage of up to about onevolt DC. For purposes of producing much larger voltages, multiple fuelcells may be assembled together to form an arrangement called a fuelcell stack, an arrangement in which the fuel cells are electricallycoupled together in series to form a larger DC voltage (a voltage near100 volts DC, for example) and to provide more power.

[0007] The fuel cell stack may include flow field plates (graphitecomposite or metal plates, as examples) that are stacked one on top ofthe other. The plates may include various surface flow field channelsand orifices to, as examples, route the reactants and products throughthe fuel cell stack. A PEM is sandwiched between each anode and cathodeflow field plate. Electrically conductive gas diffusion layers (GDLs)may be located on each side of each PEM to act as a gas diffusion mediaand in some cases to provide a support for the fuel cell catalysts. Inthis manner, reactant gases from each side of the PEM may pass along theflow field channels and diffuse through the GDLs to reach the PEM. ThePEM and its adjacent pair of catalyst layers are often referred to as amembrane electrode assembly (MEA). An MEA sandwiched by adjacent GDLlayers is often referred to as a membrane electrode unit (MEU).

[0008] A fuel cell system may include a fuel processor that converts ahydrocarbon (natural gas or propane, as examples) into a fuel flow forthe fuel cell stack. For a given output power of the fuel cell stack,the fuel flow to the stack must satisfy the appropriate stoichiometricratios governed by the equations listed above. Thus, a controller of thefuel cell system may monitor the output power of the stack and based onthe monitored output power, estimate the fuel flow to satisfy theappropriate stoichiometric ratios. In this manner, the controllerregulates the fuel processor to produce this flow, and in response tothe controller detecting a change in the output power, the controllerestimates a new rate of fuel flow and controls the fuel processoraccordingly.

[0009] A fuel cell system may include a fuel processor that converts ahydrocarbon (natural gas or propane, as examples) into a fuel flow forthe fuel cell stack. For a given output power of the fuel cell stack,the fuel flow to the stack must satisfy the appropriate stoichiometricratios governed by the equations listed above. The amount of a reactantsupplied may be referred to in terms of “stoich”. For example, for agiven electrical load on a fuel cell, one stoich of hydrogen and onestoich of air would refer to the minimum amount of each reactanttheoretically required to produce enough electrons to satisfy the load(assuming all of the reactants will react). However, in some cases, notall of the hydrogen or air supplied will actually react, so that it maybe necessary to provide excess fuel and air stoichiometry so that theamount actually reacted will be appropriate to satisfy a given powerdemand.

[0010] Hydrogen that is not reacted in the fuel cell may be vented tothe atmosphere with the fuel cell exhaust, and in some cases may beoxidized before it is vented. Such exhaust may also contain smallamounts of hydrocarbons that “slip” through the fuel processor withoutbeing reacted. Substantial heat may be generated as these exhaustcomponents are oxidized, for example by mixing them with air and passingthem through a platinum-coated ceramic monolith similar to an automotivecatalytic converter.

[0011] The fuel cell system may provide power to a load, such as a loadthat is formed from residential appliances and electrical devices thatmay be selectively turned on and off to vary the power that is demandedby the load. Thus, the load may not be constant, but rather the powerthat is consumed by the load may vary over time and abruptly change insteps. For example, if the fuel cell system provides power to a house,different appliances/electrical devices of the house may be turned onand off at different times to cause the load to vary in a stepwisefashion over time.

[0012] There is a continuing need for systems and algorithms to achieveobjectives including the foregoing in a robust and cost effectivemanner.

SUMMARY

[0013] The invention provides a combined heat and power fuel cell systemand associated methods of operation. Such systems are commonly referredto as cogeneration systems. In general, the system and methods of theinvention relate to operation of a fuel cell system among various modesand configurations to balance heat and power demand signals. The fuelcell system is coupled to both a power sink and a heat sink. Acontroller is adapted to coordinate response to data signals from thepower sink and the heat sink. As examples, such data signals from theheat sink may include a temperature indication or a heat demand signal(such as from a thermostat), and such data signals from the power sinkmay include a voltage or current measurement, an electrical power demandsignal, or an electrical load.

[0014] In one aspect, the invention provides a method of operating afuel cell system including the following steps: (1) providing a fuelflow and an oxidant flow to a fuel cell stack to produce electricity;(2) providing the electricity to an electrical load; (3) transferringheat from the fuel cell stack to a heat sink by circulating a firstcoolant through a first coolant circuit, wherein the first coolantcircuit is adapted to remove heat from the fuel cell stack and isfurther adapted to transfer heat to the heat sink; (4) measuring athermal parameter of the heat sink; (5) generating a heat demand signalwhen the thermal parameter of the heat sink is below a predeterminedlevel; and (6) shorting at least one fuel cell in the fuel cell stack inresponse to the heat demand signal. The heat sink can be a water tank, abody of air contained in a building, a generator portion of anadsorption cooling system, etc. The thermal parameter generally refersto a temperature of the heat sink.

[0015] In another aspect, the invention provides a method of operating afuel cell system, including the following steps: (1) providing a fuelflow and an oxidant flow to a fuel cell stack to produce electricity;(2) providing the electricity to an electrical load; (3) transferringheat from the fuel cell stack to a heat sink by circulating a firstcoolant through a first coolant circuit, wherein the first coolantcircuit is adapted to remove heat from the fuel cell stack and isfurther adapted to transfer heat to the heat sink; (4) measuring athermal parameter of the heat sink; (5) generating a heat demand signalwhen the thermal parameter of the heat sink is below a predeterminedlevel; and (6) selectively connecting at least two fuel cells in thefuel cell stack in parallel in response to the heat demand signal.

[0016] In one embodiment, the balance between the heat and power demandsignals is accommodated by selectively connecting at least two fuelcells within a group to increase the amount of heat that is generatedfor a given amount of power production. Where a system is adapted toselectively connect one or more cells in parallel, the cells that areselectively connected are connected via a switched network, rather thanbeing stack in series as in a conventional stack. The heat sink can be awater tank, a body of air contained in a building, a generator portionof an adsorption cooling system, etc. The thermal parameter generallyrefers to a temperature of the heat sink.

[0017] In another aspect, the invention provides a method of operating afuel cell system, including the following steps: (1) providing a fuelflow and an oxidant flow to a fuel cell stack to produce electricity;(2) providing the electricity to an electrical load; (3) transferringheat from the fuel cell stack to a heat sink by circulating a firstcoolant through a first coolant circuit, wherein the first coolantcircuit is adapted to remove heat from the fuel cell stack and isfurther adapted to transfer heat to the heat sink; (4) measuring athermal parameter of the heat sink; (5) generating a heat demand signalwhen the thermal parameter of the heat sink is below a predeterminedlevel; and (6) selectively operating the fuel cell in a low efficiencymode in response to the heat demand signal.

[0018] There are various fuel cell operating regimes that result inrelatively low efficiency operation and the production of relativelyhigh amounts of waste heat. Prior art systems are generally configuredto avoid such regimes as a means of maximizing system efficiency.However, in systems balancing both heat demand and power demand signals,it may be desirable to switch between such modes. Other examples of lowefficiency operating modes include reactant starvation, operation attemperatures outside the normal operating range of a fuel cell, andproducing a given amount of power at low voltage and high current (e.g.,cell voltages less than 0.4 volts).

[0019] Advantages and other features of the invention will becomeapparent from the following description, drawings and claims.

DESCRIPTION OF THE DRAWINGS

[0020]FIG. 1 is a schematic diagram of an integrated fuel cell system.

[0021]FIG. 2 is a schematic diagram of a control system for anintegrated fuel cell system.

[0022]FIG. 3 is a schematic diagram of an integrated fuel cell system.

[0023]FIG. 4 is a schematic diagram of a CHP fuel cell system.

[0024]FIG. 5 is a schematic diagram of a CHP fuel cell system.

[0025]FIG. 6 is a flow diagram of a control scheme for a CHP fuel cellsystem.

[0026]FIG. 7 is a flow diagram of a control scheme for a CHP fuel cellsystem.

[0027]FIG. 8 is a flow diagram of a control scheme for a CHP fuel cellsystem.

[0028]FIG. 9 is a flow diagram of a control scheme for a CHP fuel cellsystem.

DETAILED DESCRIPTION

[0029] Referring to FIG. 1, an integrated fuel cell system 100 is shown.Natural gas is injected into the system through conduit 102. The naturalgas flows through desulfurization vessel 104, which contains asulfur-adsorbent material such as activated carbon. The de-sufurizednatural gas is then flowed to a conversion reactor 110 via conduit 105.Before being reacted in the conversion reactor 110, the de-sulfurizednatural gas is mixed with air 106 and steam 108. It will be appreciatedthat the conversion reactor 110 is an autothermal reactor. The convertednatural gas, referred to as reformate, then flows through a series ofhigh temperature shift reactors 112 and 114, through a low temperatureshift reactor 116, and then through a PROX reactor 118. It will beappreciated that the primary function of this series of reactors is tomaximize hydrogen production while minimizing carbon monoxide levels inthe reformate. The reformate is then flowed via conduit 120 to the anodechambers (not shown) of a fuel cell stack 122.

[0030] Air enters the system via conduit 124 and through conduit 106 aspreviously mentioned. In the present example, the fuel cell stack 122uses sulfonated fluourocarbon polymer PEMs that need to be kept moistduring operation to avoid damage. While the reformate 120 tends to besaturated with water, the ambient air 124 tends to be subsaturated. Toprevent the ambient air 124 from drying out the fuel cells in stack 122,the air 124 is humidified by passing it through an enthalpy wheel 126,which also serves to preheat the air 124. The theory and operation ofenthalpy wheels are described in U.S. Pat. No. 6,013,385, which ishereby incorporated by reference. The air 124 passes through theenthalpy wheel 126 through the cathode chambers (not shown) of the fuelcell stack 122 via conduit 125. The air 124 picks up heat and moisturein the stack 122, and is exhausted via conduit 128 back through theenthalpy wheel 126. The enthalpy wheel 126 rotates with respect to theinjection points of these flows such that moisture and heat from thecathode exhaust 128 is continually passed to the cathode inlet air 124prior to that stream entering the fuel cell.

[0031] The anode exhaust from the fuel cell is flowed via conduit 130 toan oxidizer 132, sometimes referred to as an “anode tailgas oxidizer”.The cathode exhaust leaves the enthalpy wheel 126 via conduit 134 and isalso fed to the oxidizer 132 to provide oxygen to promote the oxidationof residual hydrogen and hydrocarbons in the anode exhaust 130. Asexamples, the oxidizer 132 can be a burner or a catalytic burner(similar to automotive catalytic converters). The exhaust of theoxidizer is vented to ambient via conduit 136. The heat generated in theoxidizer 132 is used to convert a water stream 138 into steam 108 thatis used in the system.

[0032] Referring to FIG. 2, a schematic is shown of a control system foran integrated high temperature PEM fuel cell system. Such a controlsystem can include the following components, as examples: (200) anelectronic controller, e.g., a programmable microprocessor; (202) agraphical user interface; (204) software for instructing the controller;(206) an air blower for providing the system with air, e.g., the fuelcell cathode and/or the fuel processor; (208) a fuel blower for drivinghydrocarbon into the fuel processor; (210) a stack voltage scanner formeasuring the stack voltage and/or the individual voltages of fuel cellswithin the stack; (212) a coolant pump for circulating a coolant throughthe fuel cell stack to maintain a desired stack operating temperature;(214) a coolant radiator and fan for expelling heat from the coolant toambient; (216) a fuel processor inlet air by-pass valve for controllingthe amount of air fed to the fuel processor; and (218) an oxidizer inletair control valve.

[0033] Such a control system can operate to control the followingvariables, as examples: (220) the fuel processor inlet oxygen to fuelratio; (222) the fuel processor inlet water to fuel ratio; (224) a fuelprocessor reactor temperature; (226) the voltage of the fuel cell stackor of individual fuel cells within the stack; (228) the oxidizertemperature; (230) electrical demand on the fuel cell system; (232) thecathode air stoich; (234) the anode fuel stoich; and (236) the systemcoolant temperature.

[0034] As examples, suitable fuel processor systems are described inU.S. Pat. Nos. 6,207,122, 6,190,623, and 6,132,689, which are herebyincorporated by reference. For instance, in the case of a natural gasfuel processor, the system may include a variable speed blower forinjecting natural gas into the system, and a variable speed air blowerfor injecting air into the system. The gas and air may be mixed in amixing chamber, humidified to a desired level (e.g., the system mayinclude some method of steam generation), and be preheated (e.g., in agas/gas heat exchanger with heat from product gas from the fuelprocessor). The reactant mixture may then be reacted in an autothermalreactor (ATR) to convert the natural gas to synthesis gas(H₂O+CH₄→3H₂+CO; ½O₂+CH₄→2H₂+CO). The fuel processor may also include ashift reactor (CO+H₂O→H₂+CO₂) to shift the equilibrium of the synthesisgas toward hydrogen production to minimize carbon monoxide (CO). Thefuel processor may include multiple shift reactor stages.

[0035] Some fuel processor systems may also include a preferentialoxidation (PROX) stage (CO+½O₂→CO₂) to further reduce carbon monoxidelevels. The PROX reaction is generally conducted at lower temperaturesthat the shift reaction, such as 100-200° C. Like the CPO reaction, thePROX reaction can also be conducted in the presence of an oxidationcatalyst such as platinum. The PROX reaction can typically achieve COlevels less than 100 ppm. Other non-catalytic CO reduction and reformatepurification methods are also known, such as membrane filtration andpressure swing adsorption systems.

[0036] In some embodiments, the autothermal reactor can be replaced by areforming reactor (e.g., utilizing the endothermic steam reformingreaction: H₂O+CH₄→3H₂+CO), or by a catalytic partial oxidation reactor(CPO reactor: ½O₂+CH₄→2H₂+CO), which is exothermic. These terms aresometimes used loosely or interchangeably. In general, an autothermalreactor is a reactor that combines the reforming and catalytic partialoxidation reactions to achieve a balance between the respectiveendothermic and exothermic elements. It should be noted that fuelprocessors are sometimes generically referred to as reformers, and thefuel processor output gas is sometimes generically referred to asreformate, without respect to the reaction that is actually employed.

[0037] The ATR catalyst can be a ceramic monolith that has beenwashcoated with a platinum catalyst (as known in the art, e.g.,operating at over 600° C.). The shift catalyst can also be platinumwash-coated ceramic monolith (e.g., operating between 300-600° C.). Theshift reactor can also include a catalyst that is operable at lowertemperatures. Other suitable catalyst and reactor systems are known inthe art.

[0038] In some embodiments, a desulfurization stage may be placedupstream from the fuel processor to remove sulfur compounds from thefuel before it is reacted (e.g., to avoid poisoning the catalysts of thefuel processor and/or the fuel cell stack). For example, activatedcarbon, zeolite, and activated nickel materials are all known in the artfor such application.

[0039] As known in the art, it may be desirable to control the water tofuel ratio (e.g., steam to carbon ratio) that is fed to the ATR. Forexample, it may be desirable to provide on average at least two watermolecules for every carbon atom provided in the fuel to prevent coking.It may also be desirable in some embodiments to adjust the air stoichthrough the fuel cell stack to control the amount of oxygen that isintroduced into the fuel processor with respect to the amount of fuelthat is introduced (e.g., O₂:CH₄ ratio, which can effect the operationtemperature of the ATR as an example).

[0040] Suitable fuel cell stack designs are well known. For example, thefuel cell systems taught in U.S. Pat. Nos. 5,858,569, 5,981,098,5,998,054, 6,001,502, 6,071,635, 6,174,616, and U.S. patent Ser. No.09/502,886 are each hereby incorporated by reference. In an integratedfuel cell system, the fuel cell stack may be associated with additionalcomponents and subsystems. A coolant system may be used to circulate aliquid coolant through the stack to maintain a desired operatingtemperature. A radiator or other heat transfer device may be placed inthe coolant path to provide coolant temperature control. The coolant mayalso perform heat transfer in other areas of the system, such as in thefuel processor, or cooling reactants exiting the fuel processor to adesired temperature before entering the fuel cell stack. As an example,the coolant may be circulated by a variable speed pump.

[0041] The reactant delivery system associated with the fuel cell stackmay include a variable speed air blower or compressor, and variableposition valves and/or orifices to control the amount and pressure offuel and air provided to the stack, as well as the ratio between thetwo. For a given electrical load, a certain amount of reactants must bereacted in the fuel cell to provide the power demanded by the load. Inthis sense, the amount of air and fuel supplied to the fuel cell stackmay each be referred to in terms of stoichiometry (i.e., thestoichiometric equations associated with the fuel cell reactions:H₂→2H⁺+2e⁻; and O₂+4H⁺+4e⁻→2H₂O). For example, supplying 1 “stoich” ofreformate means that enough reformate is supplied to the fuel cell stackto satisfy the power demand of the load, assuming that all of thehydrogen in the reformate reacts. However, since not all of the hydrogenin the reformate will actually react, the fuel may be supplied at anelevated stoich (e.g., 2 stoich would refer to twice this amount) toensure that the amount that actually will react will be enough to meetthe power demand. Similarly, air may also be supplied to the fuel cellstack in excess of what is theoretically needed (e.g., 2 stoich).

[0042] The reactant plumbing associated with the stack may be conductedin part by a manifold. For example, the teachings of U.S. patent Ser.No. 09/703,249 are hereby incorporated by reference. Such a manifold maybe further associated with a water collection tank that receivescondensate from water traps in the system plumbing. The water tank mayinclude a level sensor. Some fuel cell systems may require an externalsource of water during operation, and may thus include a connection to amunicipal water source. A filter may be associated with the connectionfrom the municipal water supply, such as a particulate filter, a reverseosmosis membrane, a deionization bed, etc.

[0043] Some fuel cell membranes, such as those made from sulfonatedflourocarbon polymers, require humidification. For example, it may benecessary to humidify reactant air before it is sent through the fuelcell in order to prevent drying of the fuel cell membranes. In suchsystems, a reactant humidification system may be required. It will beappreciated that in systems utilizing reformate, this generally refersto humidifying only the air fed to the fuel cell stack and not the fuelstream, since the reformate exiting the fuel processor is generallysaturated. One method of humidification is to generate steam which issupplied to a reactant stream. Membrane humidification systems are alsoknown, as well as enthalpy wheel systems, as taught in U.S. Pat. No.6,013,385, which is hereby incorporated by reference.

[0044] The spent fuel exhausted from the fuel cell stack may containsome amount of unreacted hydrogen or unreacted hydrocarbon or carbonmonoxide from the fuel processor. Before the spent fuel is vented to theatmosphere, it may be sent through an oxidizer to reduce or remove suchcomponents. Suitable oxidizer designs are known, such as bumer designs,and catalytic oxidizers similar to automotive catalytic converters.Oxidizers may utilize air exhausted from the fuel cell stack, and mayhave an independent air source, such as from a blower. In some systems,the heat generated by the oxidizer may be used, for example, to generatesteam for use in the fuel processor or to humidify the fuel cellreactants. Exemplary oxidizer designs are described in U.S. patent Ser.Nos. 09/727,921 and 09/728,227, which are each incorporated herein byreference. Fuel cell exhaust oxidizers are sometimes referred to as“tailgas oxidizers” or “anode tailgas oxidizers” (“ATO”).

[0045] Another system that may be associated with the fuel cell stack isa mechanism for measuring the voltages of the individual fuel cellswithin the stack. For example, the teachings of U.S. Pat. No. 6,140,820,U.S. patent Ser. Nos. 09/379,088, 09/629,548, 09/629,003 are each herebyincorporated by reference. In some systems, the health of a fuel cellstack may be determined by monitoring the individual differentialterminal voltages (also referred to as cell voltages) of the fuel cells.Particular cell voltages may individually vary under load conditions andcell health over a range from −1 volt to +1 volt, as an example. Thefuel cell stack typically may include a large number of fuel cells(between 50-100, for example), so that the terminal voltage across theentire stack is the sum of the individual fuel cell voltages at a givenoperating point. As the electrical load on the stack is increased, some“weak” cells may drop in voltage more quickly than others. Driving anyparticular cell to a low enough voltage under an electrical load candamage the cell, so systems may include a mechanism for coordinating thecell voltages with the electrical demand and reactant supply to the fuelcell stack. For example, the teachings of U.S. patent Ser. Nos.09/749,261, 09/749,297 are hereby incorporated by reference.

[0046] A fuel cell stack typically produces direct current at a voltagewhich varies according to the number of cells in the stack and theoperating conditions of the cells. Applications for the power generatedby a fuel cell stack may demand constant voltage, or alternating currentat a constant voltage and frequency similar to a municipal power grid,etc. Integrated fuel cell systems may therefore include a powerconditioning system to accommodate such demands. Technologies forconverting variable direct current voltages to constant or relativelyconstant voltages are well known, as are technologies for invertingdirect currents to alternating currents. Suitable power conditionertopologies for fuel cells are also well known. For example, theteachings of U.S. patent Ser. No. 09/749,297 are hereby incorporated byreference.

[0047] A battery system may also be associated with the powerconditioning system, for example, to protect the fuel cells from fuelstarvation upon sudden electrical load increases on the stack. A batterysystem can also be used, as examples, to supplement the peak outputpower of the fuel cell system, or to provide continuous power to anapplication while the fuel cell system is temporarily shut down (as forservicing) or removed from the load. The battery system may also includea system for periodically charging the batteries when necessary.

[0048] Some fuel cell systems may be operated independently from thepower grid (grid independent systems), while other fuel cell systems maybe operated in conjunction with the power grid (grid parallel systems).For grid parallel systems, the system may include a transfer switch totransfer the electrical load between the fuel cell system and the powergrid. For example, in some grid parallel systems, the electrical loadcan be switched from the fuel cell system to the grid when the fuel cellsystem needs to be shut down for maintenance. In still other gridparallel systems, the electrical load can be shared between the fuelcell system and the grid. The fuel cell can also be used to feed powerto the grid (in this sense, the grid may be referred to as a “sink”),while an appliance takes its power from the grid. Other arrangements arepossible.

[0049] System controllers may automate the operation of fuel cellsystems to varying degrees, and may have varying capacities foradjustment and reconfiguration. For example, some controllers may relyin part on software for instruction sets to provide enhanced flexibilityand adaptability, while other controllers may rely on hardware toprovide enhanced reliability and lower cost. Control systems may alsoinclude combinations of such systems. Controllers may include analgorithm that coordinates open and closed loop functions. In thiscontext, an open loop function is one that does not utilize feedback,such as adjusting a blower according to a look-up table withoutverifying the effect of the adjustment or iterating the adjustmenttoward a desired effect. A closed loop function is one that utilizesfeedback to iterate adjustments toward a desired effect.

[0050] In general, the controller circuitry may include data inputs fromsystem components such as safety sensors and thermocouples throughoutthe system. As an example, such data inputs may report data in the formof variable voltage or current signals, or as binary on/off signals. Thecontroller circuitry may also include devices to control the voltageand/or current supplied to various components in the system, for exampleto control variable speed pumps and blowers. The power supplied tosystem components may be referred to as the parasitic load.

[0051] Referring to FIG. 3, a schematic diagram is shown of anintegrated fuel cell system 300. A fuel processor 302 receives air 304,steam 306 and natural gas 308. Similar to the fuel processor discussedwith respect to FIG. 1, the fuel processor 302 converts the reactantstreams 304, 306 and 308 into a reformate stream 310 that is flowedthrough a fuel cell stack 312 where it reacted at the anode electrodesof the fuel cells in stack 312. The fuel cell stack 312 also receives aflow of air via conduit 314 that provides oxygen that is reacted at thecathode electrodes of the fuel cell stack 312.

[0052] The spent reformate is exhausted from fuel cell 312 via conduit316 and is fed to oxidizer 318 to remove any carbon monoxide, hydrogen,or residual hydrocarbons in the exhaust. The oxidizer 318 is a catalyticoxidizer similar to an automotive catalytic converter. The oxidizerreceives its oxygen via conduit 320, which channels the air exhaustedfrom the fuel cell stack 312. In some embodiments, the oxidizer 318 canfurther receive a supplemental supply of oxygen to ensure adequateoxygen to oxidize combustibles in the fuel exhaust 316. In otherembodiments, excess air stoich can be supplied to the fuel cell stack312 to ensure that the cathode exhaust 320 has sufficient oxygen.

[0053] The system 300 includes various thermal management aspects. Acoolant is circulated through the fuel processor 302 via inlet 322 andoutlet 324. In this example, an outlet of a high temperature shiftreactor is cooled in heat exchanger 326 from about 600° C. to about 300°C. and the reformate is then provided to a low temperature shiftreactor. The reformate exits the fuel processor at a temperature ofabout 200° C., and is cooled in heat exchanger 332 to the operatingtemperature of the stack 312 (e.g., about 60-80° C.). Heat is generatedas the fuel cell is operated, and the operating temperature of the stack312 is maintained by circulating a coolant through the stack 312 viainlet 328 and outlet 330.

[0054] Heat is recovered from the exhaust 334 of the oxidizer 318 inheat exchanger 336. For example, in some embodiments, a coolant can becirculated through heat exchanger 336 to transfer heat from the oxidizerto another part of the system 300, such as to preheat the air 304 andfuel 308 streams fed to the fuel processor 302. In other embodiments,water can be flowed through heat exchanger 336 to generate steam, whichis then used to humidify the air stream 314 that is fed to the fuel cellstack, or to provide the steam flow 306 that is used in the fuelprocessor 302.

[0055] Referring to FIG. 4, a schematic diagram is shown of a CHP fuelcell system 400. A system coolant is circulated through a fuel cellsystem 402 to transfer heat from the fuel cell system 402 to a heat sink408. In this example, the heat sink 408 is a hot water tank that wouldbe used to provide hot water via conduit 409 to a building such as ahome or an apartment. The coolant is circulated out of the fuel cellsystem 402 via conduit 404, through a heat exchanger 410, and then backinto fuel cell system 402 via conduit 406. A pump inside the fuel cellsystem drives the coolant flow.

[0056] A coolant is also circulated from the heat exchanger 410 viaconduit 412 to heat exchange surface 414 in heat sink 408, and then backto heat exchanger 410 via conduit 416. The coolant flow is driven bypump 418. In some embodiments, the heat exchanger 410 can be located inheat sink 408 such that a single coolant loop is circulated between fuelcell system 402 and heat sink 408. However, the embodiment shownprovides an advantage in that the equipment associated with the heatsink can be configured independently form the fuel cell system 402.

[0057] For example, the heat sink 408 also includes a level sensor 420and a temperature sensor 422 (e.g., a thermostat). The level sensor 420serves to ensure that the water level in tank 408 stays above apredetermined threshold. For example, a valve can be actuated to allow atap water line to fill the tank 408 to make up for water flowed out ofthe tank 408 via conduit 409. The temperature sensor 422 serves toensure that the temperature of the water in tank 408 stays above adesired level.

[0058] The temperature sensor 422 can be connected to a controller thatis further connected to pump 418. As an example, where the temperaturesensor 422 indicates heat is needed to bring the temperature of the tank408 to a desired level, the pump 418 can be turned on to transfer heatfrom the heat exchanger 410 to the tank 408. Where no heat is needed inthe tank, the pump 418 can remain off such that no heat is transferredfrom the heat exchanger 410 to the tank 408. In such a case, the fuelcell system 402 may continue to circulate coolant through the heatexchanger 410, but may also operate a radiator in the fuel cell system402 to expel heat to ambient to maintain the operating temperatures(e.g., the coolant temperature) in the fuel cell system 402 at desiredlevels.

[0059] In some embodiments, the heat exchanger 410 can be located withinthe fuel cell system 402. For example, the fuel cell system 402 caninclude an inlet and an outlet hook-up for a heat sink such as a watertank. The flow through this circuit can be provided by a pump in thefuel cell system, or by a pump at the heat sink location. It will beappreciated that many variations are possible.

[0060] Referring to FIG. 5, a schematic diagram is shown of anotherexample of a CHP fuel cell system 500. A coolant is circulated between afuel cell system 502 and a radiator 508 to maintain the operatingtemperatures (e.g., the coolant temperature) in the fuel cell system 502at desired levels. The coolant flows from the fuel cell system 502 tothe radiator 508 via conduit 504 and returns from the radiator 508 tothe fuel cell system 502 via conduit 506. A blower 509 is associatedwith the radiator 508 that is actuated to cool the coolant flowedthrough the radiator 508 by blowing a relatively cool fluid across theradiator 508. In this example, the blower is used to flow cold air frombuilding 514 via conduit 512 across the radiator 508, where the air isheated and is then flowed back to building 514 via conduit 512. Theblower 509 may be actuated, as an example, by a thermostat located inbuilding 514. The radiator 508 may also include a second blower toreject radiator heat to ambient to maintain a desired operatingtemperature of the fuel cell system 502 when heat is not required by thebuilding 514. This example illustrates another means by which fuel cellsystem heat can be provided to a heat sink. It will be appreciated thatwhile the heat sink in this example is generally the building 514, itcould also be defined in terms of the radiator 508 as a matter ofperspective.

[0061] In one aspect of the invention, an integrated fuel cell systemincludes a fuel processor, a fuel cell stack, a power conditioningsystem, and a control scheme adapted to coordinate the operation ofthese systems with at least one heat sink and at least one power sink.The terms “integrated fuel cell system” and “fuel cell system” are usedinterchangeably, and generally refer to a fuel cell stack that iscoupled to components and subsystems that support the operation of thestack and the application of power generated by the stack. The termcombined heat and power (“CHP”) fuel cell system refers to a fuel cellsystem that is used to provide both power and the utilization of wasteheat. For example, a fuel cell can be used to produce electricity, andwaste heat from the fuel cell system can be used for variousapplications where heat is needed (e.g., adsorption coolers, waterheaters, boilers, furnaces, etc.) to reduce the fuel or electricityordinarily required by such applications. Such systems are alsosometimes referred to as “co-generation” or “co-gen” systems. Theutilization of waste heat can dramatically increase the efficiency ofsuch systems.

[0062] The fuel processor includes a first coolant system wherein heatfrom exothermic reactions within the fuel processor is transferred to acoolant fluid to maintain a desired temperature in at least a portion ofthe fuel processor. The coolant system may be adapted to simultaneouslymaintain various different temperatures within the fuel processor, forexample, by varying coolant flow, heat transfer surface area, andreactant flow within the fuel processor associated with a given heatexchanger. As an example, the coolant fluid may be glycol-based, such aspropylene glycol. Exemplary fuel processor systems are described in U.S.Pat. Nos. 6,207,122, 6,190,623, and 6,132,689, the teachings of whichare each hereby incorporated herein by reference.

[0063] Suitable fuel cell stack designs are well known. For example, thefuel cell systems taught in U.S. Pat. Nos. 5,858,569, 5,981,098,5,998,054, 6,001,502, 6,071,635, 6,174,616, and U.S. patent Ser. No.09/502,886 are each hereby incorporated by reference. A fuel cell stackmay also be incorporated that is based on a “high temperature” PEM, suchas the polybenzimidazole (“PBI”) fuel cell membranes manufactured byCelanese. U.S. patents describing this material include U.S. Pat. Nos.5,525,436, 6,099,988, 5,599,639, and 6,124,060, which are eachincorporated herein by reference. In this context, “high temperature”PEM's generally refer to PEM's that are operated at temperatures over100° C. (e.g., 150-200° C.). Stacks based on other high temperaturemembrane materials such as polyether ether ketone (“PEEK”) may also besuitable.

[0064] The stack includes a second coolant system that is adapted tomaintain a desired operating temperature of the stack. As an example, acoolant fluid may be circulated through coolant channels between eachfuel cell in the stack. It is generally desirable that a coolant flowingthrough the stack be substantially dielectric to prevent the coolantfrom shorting the fuel cells in the stack. This issue may also beaddressed in other ways, for example, by electrically isolating thecoolant as it flows through the stack. As examples, the coolant can bedeionized water or glycol. In some cases, the first coolant systemassociated with the fuel processor may be the same as the second coolantsystem associated with the stack. For example, a common fluid may beflowed through both systems. In other cases, the coolant systems may bedistinct, and may contain different coolant fluids. For example, acoolant such as glycol with a relatively high boiling point may be usedin the fuel processor, whereas a coolant with a relatively low boilingpoint such as water may be used in the fuel cell stack.

[0065] A third coolant system may be associated with an oxidizing unitthat is adapted to oxidize combustible components in the fuel cellexhaust such as hydrogen and unreacted hydrocarbons before the exhaustis vented to the atmosphere. As an example, such an oxidizing unit mayresemble an automotive catalytic converter unit and be maintained at atemperature over 600° C. Maintaining the oxidizer temperature mayrequire sinking a substantial amount of heat from the exothermicoxidation into the third coolant system, depending on factors such asthe level of excess hydrogen that is fed to the fuel cell stack, theamount of residual hydrocarbons in the fuel cell exhaust and the amountof air that is supplied to oxidize these components. In some cases, thethird coolant system may be a portion of the first or second coolantsystems.

[0066] A fuel cell stack typically produces direct current at a voltagewhich varies according to the number of cells in the stack and theoperating conditions of the cells. Applications for the power generatedby a fuel cell stack may demand constant voltage, or alternating currentat a constant voltage and frequency similar to a municipal power grid,etc. Integrated fuel cell systems as in the present invention maytherefore include a power conditioning system to accommodate suchdemands. Technologies for converting variable direct current voltages toconstant or relatively constant voltages are well known, as aretechnologies for inverting direct currents to alternating currents.Suitable power conditioner topologies for fuel cells are also wellknown. For example, the teachings of U.S. patent Ser. No. 09/471,759 arehereby incorporated by reference.

[0067] A battery system may also be associated with the powerconditioning system, for example, to protect the fuel cells from fuelstarvation upon sudden electrical load increases on the stack. A batterysystem can also be used, as examples, to supplement the peak outputpower of the fuel cell system, or to provide continuous power to anapplication while the fuel cell system is temporarily shut down (as forservicing) or removed from the load. The battery system may also includea system for periodically charging the batteries when necessary.

[0068] Some fuel cell systems may be operated independently from thepower grid (grid independent systems), while other fuel cell systems maybe operated in conjunction with the power grid (grid parallel systems).For grid parallel systems, the system may include a transfer switch totransfer the electrical load between the fuel cell system and the powergrid. For example, in some grid parallel systems, the electrical loadcan be switched from the fuel cell system to the grid when the fuel cellsystem needs to be shut down for maintenance. In still other gridparallel systems, the electrical load can be shared between the fuelcell system and the grid. Other arrangements are possible.

[0069] The heat sink of the present system is a media to which heat fromany of the above described coolant systems is transferred. In somesystems having multiple distinct coolant loops, heat from one coolantloop may be transferred to another coolant loop having a lowertemperature by way of a liquid-to-liquid heat exchanger. Thus, the heatsink may receive heat from throughout the fuel cell system whiledirectly contacting only one of the coolant loops. In other embodiments,each coolant loop may be associated with a heat sink. In still otherembodiments, each coolant loop can be associated with multiple heatsinks.

[0070] In some systems, the coolants are flowed through an air-cooledradiator, such that heat from the fuel cell system is transferred to theair around the fuel cell system. In this example, the radiator is theheat sink. In other embodiments, the heat sink can be a liquid-to-liquidheat exchanger, for example to exchange heat from a liquid fuel cellsystem coolant to a hot water tank or to an external fluid looptransferring heat to a hot water tank. The fuel cell system coolant canalso be in vapor form as it contacts a heat sink. In such cases, in someembodiments the heat sink can serve as a condenser of the vaporizedcoolant. Also, in some embodiments, the temperatures of fuel cell systemcomponents can be regulated by direct interaction with heat sinks ratherthan by passing heat to a coolant loop and then to the heat sink. Thepresent invention contemplates that heat from the fuel cell system canbe transferred to a number of applications where heat is desired,including domestic and commercial hot water tanks, and air cooledradiators that supply heat to buildings or other applications.

[0071] The various sources of heat within the fuel cell system maydiffer in terms of temperature and amount of heat. For example, fuelcells are generally operated at temperatures much lower than fuelprocessors, such that the temperature of the waste heat from a fuel cellstack is generally lower than that of the waste heat from a fuelprocessor.

[0072] Waste heat at relatively lower temperatures is sometimes referredto as “lower grade” heat, whereas waste heat at relatively highertemperatures can be referred to as “higher grade” heat. This is due tothe fact that heat transfer efficiency is generally greater when heat istransferred across relatively large temperature differences. Likewise,it is generally less efficient to transfer heat between masses having arelatively small temperature difference. Thus, the applicability of aparticular waste heat stream for heat recovery may vary according to thetemperature and mass flow of the waste heat stream, and according to thetemperature and mass (or mass flow) into which the waste heat istransferred.

[0073] A power sink of the present invention may be any application towhich the power generated by the fuel cell stack is sent. For example,the fuel cell system may be used to power residential appliances (e.g.,110VAC, 60 Hz). The fuel cell system may also be used to feed power to autility grid. Other applications may require direct current power fromthe fuel cell system. Finally, the “parasitic load” of any or all of theelectric components within the fuel cell system (e.g., valves, pumps,blowers, controllers, etc.) can also represent a power sink for the fuelcell system.

[0074] System controllers may automate the operation of fuel cell systemcomponents to varying degrees, and may have varying capacities foradjustment and reconfiguration. For example, some controllers may relyin part on software for instruction sets to provide enhanced flexibilityand adaptability, while other controllers may rely on hardware toprovide enhanced reliability and lower cost. Control systems may alsoinclude combinations of such systems. Controllers may include analgorithm that coordinates open and closed loop functions. In thiscontext, an open loop function is one that does not utilize feedback,such as adjusting a blower according to a look-up table withoutverifying the effect of the adjustment or iterating the adjustmenttoward a desired effect. A closed loop function is one that utilizesfeedback to iterate adjustments toward a desired effect.

[0075] In general, the controller circuitry may include data inputs fromsystem components such as safety sensors and thermocouples throughoutthe system. As an example, such data inputs may report data in the formof variable voltage or current signals, or as binary on/off signals. Thecontroller circuitry may also include devices to control the voltageand/or current supplied to various components in the system, for exampleto control variable speed pumps and blowers. In general, the logicemployed by a system controller may be referred to as a control scheme.In some cases, fuel cell systems can include multiple independentcontrollers and control schemes, that may or may not be coordinated by acommon controller or control scheme.

[0076] In one aspect of the invention, a control scheme is provided fora combined heat and power fuel cell system that coordinates control ofthe system between a heat sink and a power sink. For example, if thepower sink is a set of residential appliances and the heat sink is a hotwater heater for the residence, the demand for power may be independentfrom the demand for hot water. However, the amount of power and wasteheat available from the system are linked because the waste heat is aby-product of operating the system to produce power. Still, there may besituations where power is required, but little or no heat is needed, orvice versa. There is thus a need to efficiently coordinate and balancethe operation of the system between such demands.

[0077] In one embodiment, the hydrogen stoich is adjusted to meet agiven power demand. The hydrogen stoich may be minimized such that nomore hydrogen is supplied to the fuel cell stack than is required tomeet the electrical load. For example, the teachings of U.S. patent Ser.No. 09/749,298 are hereby incorporated by reference. Minimizing thehydrogen stoich in this way increases the efficiency of the system(without respect to efficiency gains from waste heat recovery). Whenhydrogen stoich is supplied in excess of what is needed to meet theelectrical load on the fuel cells, the excess hydrogen simply passesthrough the stack unreacted and is oxidized in the ATO. Thus, under oneembodiment of the present invention, when there is no heat demand (e.g.,from a thermostat on a hot water heater), the system provides onlyenough hydrogen stoich to meet the electrical load on the system. Whenheat is demanded from the system, the hydrogen stoich is increased sothat excess hydrogen is provided to the ATO, which generates heat thatis transferred to the heat sink. The heat demand signal can be binary(on/off), or it can be dynamic to increase the heat output to a desiredlevel.

[0078] As an example, referring to FIG. 6, a flow diagram is shown of acontrol scheme 600 for a CHP fuel cell system. In a first step 602, thesystem determines if there is a power demand signal indicating anelectrical load on the fuel cell system. If there is no power demandsignal, then the system maintains an idle function 604 an continueschecking for a power demand signal. In the event of a power demandsignal, the system then determines in a step 606 whether the poweroutput of the fuel cell system is adequate to meet the electrical loadplaced on the system. If the power output is not adequate, then thereactant flow rates are incrementally increased in step 608 and thesystem returns to step 606. If the power output is adequate, the systementers a stoich optimization mode 610. The stoich optimization mode 610can include, for example, reducing the fuel flow to the fuel cell untilthe voltage or some other performance parameter of the fuel cell (or ofthe weakest cell in a stack) is affected to an unacceptable extent. Forexample, to avoid damaging the fuel cell stack from reactant starvation,it may be desirable to monitor all of the cell voltages in the stack,and to maintain reactant flow rates high enough to prevent any of thecells from dropping below 0.4 volts. In some embodiments, excess airflow is maintained such that only the flow of the fuel is modulated bythe algorithm 600.

[0079] The system also performs a check 612 for a heat demand signal.For example, the heat demand signal could be a thermostat indicatingthat a water tank is below a desired temperature. In the example shownin FIG. 6, the fuel cell system heat used to supply the heat in responseto a heat demand signal is supplied primarily from an oxidizer unit (seeheat exchanger 336 of FIG. 3). If there is no heat demand signal, thesystem continues in optimization mode 610. Where there is a heat demandsignal, the system then performs an increase 614 in the reactant flowrates. For example, in this example, for a constant power demand,increasing the fuel flow rate will increase the amount of unreacted fuelin the fuel cell exhaust that is processed in the oxidizer to generateheat.

[0080] In the next step 616, the system checks whether the heat beingsupplied by the system to the heat sink is adequate to meet the demandfor heat. For example, step 616 could include, as examples, calculatingwhether enough heat will be made available in a desired amount of timeat a given operating point, or it can include supplying heat at a givenoperating point for a period of time and then checking again whether thedesired amount of heat has been supplied. If not enough heat has beensupplied, then the system further increases the flow rates of thereactants. If the heat demand has been met, the system returns 618 toits optimization mode. It will be appreciated that the system iscontinually looking for power or heat demand signals by repeatedlycycling through these determinations. Some embodiments may eliminate thedetermination 616 and simply continue producing a heat demand signalwhile heat is needed.

[0081] In another embodiment, the hydrogen stoich is similarly minimizedsuch that no more hydrogen is supplied to the fuel cell stack than isrequired to meet the electrical load. When heat is required from thesystem, a heat demand signal causes raw hydrocarbon (e.g., natural gasor propane) to bypass the fuel processor so that it is oxidized in theATO to provide the desired heat. The heat demand signal can be binary(on/off), or it can be dynamic to increase the heat output to a desiredlevel.

[0082] Referring to FIG. 7, another flow diagram 700 is shown of acontrol scheme for a CHP fuel cell system to illustrate various logicaloptions that may be implemented by a system to balance a combination ofheat and power demand signals. In a first state 702, there is a powerdemand, but no heat demand. In response, the system lowers the reactantflow rates in step 704 to a point where the power demand can still bemet. Step 704 serves to maximize fuel efficiency. In this mode, thesystem also exhausts its waste heat to ambient in a step 706 (e.g., theenvironment outside the fuel cell system, or to the atmosphere).

[0083] In a second state 708, there is both a power demand and a heatdemand. In this state 708, the system increases reactant flow rates in astep 710 until the power and heat demands are both met. It will beappreciated that the power demand will be met first, and the heat demandwill then be met as the excess reactants reach a point where the energyproduced by oxidizing the excess fuel is sufficient to meet the heatdemand.

[0084] In a third state 712, there Is no power demand, but there is aheat demand. In this state 712, the system can be configured to selectbetween two options. In a first response option 714, the fuel cellsystem is maintained at a constant power output (or an output thatdirectly tracks an electrical load), and fuel is bypassed from the fuelprocessor directly to the oxidizer to produce the heat required. In somecases, it may be desirable to continue operating the fuel processor andto instead bypass the reformate from the fuel cell stack to theoxidizer. In a second response option under state 712, the systemoperates the fuel cell at a power output sufficient to provide enoughwaste heat to meet the heat demand. The excess power produced is thenput into a power sink, such as in charging a battery system or supplyingpower to a utility grid.

[0085] In a fourth state 718, there is no power demand and no heatdemand. In this example, the system responds in step 720 by idling,meaning that the system operates at just a high enough power output tomaintain readiness for general operation. The power produced is sent toa power sink.

[0086] In another embodiment, the heat sink receives at least a portionof its heat from the fuel cell stack (e.g., from a coolant circulatedthrough the stack and contacted with the heat sink). In this embodiment,the system responds to a heat demand signal (e.g., from a thermostat ona hot water heater) by shorting at least one fuel cell within the fuelcell stack. For example, the teachings of U.S. patent Ser. No.09/428,714 are hereby incorporated by reference. When a fuel cell isshorted, its electrical potential is driven to zero and all powergenerated in the cell is in the form of heat. Essentially, the shortedcell is converted into a resistive heater. In this way, additional heatcan be supplied by the fuel cell system for a given power output of thefuel cell stack.

[0087] For example, referring to FIG. 8, a flow diagram 800 is shown ofa control scheme for a CHP fuel cell system. This method of operationcontains the following logical steps: (802) operating the system at areactant stoichiometry optimized according to the power demand; (804)checking for a heat demand signal; (806) shorting at lest one fuel cellwhen there is a heat demand signal; (808) checking whether the heatdemand has been met; (810) deactivating the cell shorting mechanism whenthe heat demand has been met; and returning to step (802) to repeat thesteps (802)-(810).

[0088] Such a control scheme may also include a step where the hydrogenstoich is minimized with respect to the electrical load, and a stepwhere the reformer output to the fuel cell stack is increased over whatwould normally be supplied for a given electrical load to compensate forthe loss of power production of any cells that are shorted. Similarly,an additional step in the control logic may be provided where the powerconditioning system compensates for the reduction in voltage from thestack resulting from having some cells shorted (e.g., a DC to DCconversion operation is modified to provide a higher voltage to a DC toAC inverter). In some systems, the power conditioning components maytolerate a range of input voltages such that such a step is unnecessary(e.g., a voltage tolerant or multi-voltage inverter).

[0089] Such a control scheme may also include a step where the operatingconditions of the un-shorted cells are adapted to optimize the currentdensity and/or voltage of the un-shorted cells. For example, it is wellunderstood in the art that a fuel cell voltage and current density canbe manipulated according to the electrical load on the cell and thereactant conditions and stoichiometry provided. For example, theteachings of U.S. patent Ser. No. 09/471,759 (referenced above) includethe use of a battery system coupled with a dynamic current limitingdevice to supply constant power to an electrical load while controllingthe portion of the load that is placed on the fuel cell stack.

[0090] In another embodiment, the heat sink also receives at least aportion of its heat from the fuel cell stack. The fuel cells in thesystem are divided into at least two sections of cells connected inseries. In a first operating mode, the sections of cells are connectedin series, and in a second operating mode, at least two sections ofcells are operated in parallel. In general, the first and secondoperating modes will provide different operating efficiencies in termsof the amount of heat produced per unit power. For example, the secondoperating mode may produce more heat. In such an embodiment, a heatdemand signal may result in the system switching to the operating modeproviding the most heat for the amount of power produced. Suchembodiments may include the use of a power conditioning system capableof accommodating the differing voltages associated with each operatingmode.

[0091] In another embodiment, the heat sink also receives at least aportion of its heat from the fuel cell stack. In this embodiment, tovary the amount of heat that is produced by the fuel cell stack for agiven power output, the reactant stoichiometry is reduced until therespective voltages of the fuel cells in the stack are reduced to apoint where the stack begins producing more heat with respect to theamount of power produced. Such embodiments may include the use of apower conditioning system capable of accommodating the resultingvariation in stack voltage. Also, such embodiments may include the useof MEA's in the stack that are tolerant to reactant starvation underload. For example, the teachings of U.S. patent Ser. No. 09/727,748 arehereby incorporated by reference.

[0092] Referring to FIG. 9, a flow diagram is shown of a control schemefor a CHP fuel cell system. This method of operation contains thefollowing logical steps: (902) operating the system at a reactantstoichiometry optimized according to the power demand; (904) checkingfor a heat demand signal;

[0093] (906) activating a low efficiency operating mode where waste heatis increased for a given power output; (908) checking whether the heatdemand has been met; (910) deactivating the low efficiency operatingmode when the heat demand has been met; and returning to step (902) torepeat the steps (902)-(910).

[0094] In another aspect, the invention provides a control apparatus forexecuting any of the above logic schemes for coordinating the power andheat output of a fuel cell system. Techniques for preparing circuitry toprovide electronic control systems are well known in the art, such thata system under the present invention with the features and aspectsdescribed above could be implemented by one of ordinary skill, forexample by reference in part to the patents mentioned above.

[0095] In another aspect of the invention, a method is provided forenabling a fuel cell system to accommodate variable demands for heat andpower output. In one embodiment, the method includes providing excessfuel to the fuel cell system in response to a control signal (e.g., aheat demand signal as from a thermostat) such that the excess unreactedfuel is burned in a fuel cell exhaust oxidizer to produce heat. Inanother embodiment, the method includes shorting at least one fuel cellwithin the fuel cell stack in response to a control signal to provideadditional heat into a fuel cell stack coolant fluid. In anotherembodiment, the method may include selectively electrically connectingfuel cells in a low efficiency mode (e.g., some cells in parallel ratherthan in series) in response to a control signal (e.g., a heat demandsignal as from a thermostat) to provide additional heat into a fuel cellstack coolant fluid. In another embodiment, the method may includeselectively fuel starving a fuel cell under load to an operating pointproviding a desired balance between power and heat production of thefuel cell. In each of these embodiments, the method may further includeflowing or selectively flowing a fuel cell system coolant to a heat sinkto transfer fuel cell system waste heat to a heat sink.

[0096] In another aspect of the invention, an integrated fuel cellsystem is provided that is coupled to a power sink and a heat sink. Acontroller of the fuel cell system is adapted to respond to data signalsfrom the power sink and the heat sink. For example, such data signalsfrom the heat sink may include a temperature indication or a heat demandsignal (such as from a thermostat). Such data signals from the powersink may include a voltage or current measurement, an electrical powerdemand signal, or an electrical load.

[0097] In one embodiment, the controller is adapted to provide excessfuel to the fuel cell system in response to a control signal (e.g., aheat demand signal as from a thermostat) such that the excess unreactedfuel is burned in a fuel cell exhaust oxidizer to produce heat. Inanother embodiment, the controller is adapted to activate a mechanism toshort at least one fuel cell within the fuel cell stack to provideadditional heat into a fuel cell stack coolant fluid. In anotherembodiment, the controller is adapted to selectively electricallyconnect fuel cells in a low efficiency mode (e.g., some cells inparallel rather than in series) to provide additional heat into a fuelcell stack coolant fluid. In another embodiment, the controller isadapted to selectively fuel starve a fuel cell under load to anoperating point providing a desired balance between power and heatproduction of the fuel cell. In each of these embodiments, thecontroller may be further adapted to direct a fuel cell system coolantto a heat sink to transfer fuel cell system waste heat to a heat sink.

[0098] In another embodiment of the invention, an article of manufactureis provided that includes at least one computer usable medium havingcomputer readable code embodied thereon for enabling the coordination ofheat demand and power demand signals in the operation of an integratedCHP fuel cell system. For example, such code may implement any of thelogic operations and functions described above, by themselves or incombination.

[0099] In another embodiment, the invention provides at least oneprogram storage device readable by a machine, tangibly embodying atleast one program of instructions executable by the machine to perform amethod for enabling a fuel cell system to accommodate simultaneousvariable demands for heat and power output, including any of thefeatures described above, by themselves or in combination.

[0100] In another embodiment, a fuel cell system is provided thatincludes a fuel cell stack and a first coolant circuit. The firstcoolant circuit is adapted to circulate a first coolant through the fuelcell stack and transfer heat from the fuel cell stack to a heat sink. Aspreviously discussed, the heat sink can be any medium or object thatheat is transferred to. In some embodiments, the heat sink is a hotwater tank. In other embodiments, the heat sink is a body of air in abuilding. In still other embodiments, the heat sink is a generatorportion of an adsorption cooling system. Other heat sink applicationsare possible.

[0101] A second heat source such as a fuel processor or an exhaust gasoxidizer is present in the system and a second coolant circuit isadapted to circulate a second coolant through the second heat source totransfer heat from the second heat source to the heat sink. A controlleris connected to a first pump and adapted to vary an output of the firstpump, wherein the first pump is located in the first coolant circuit todrive the first coolant flow. A second pump is also connected to thecontroller, which is adapted to vary an output of the second pump, andwherein the second pump is located in the second coolant circuit todrive the second coolant flow. As examples, the controller can beadapted to maintain a temperature of the fuel cell stack above or belowa predetermined level, or to maintain a temperature of the second heatsource above or below a predetermined level.

[0102] In some embodiments, the heat sink is a heat exchanger includinga first flow path adapted to receive a flow of the first coolant, asecond flow path adapted to receive a flow of the second coolant; and athird flow path adapted to receive a flow of a third fluid. For example,the heat exchanger could receive cold water as the third fluid. In oneportion of the heat exchanger, the water receives heat from the firstfluid (e.g., fuel cell coolant) and in a second portion of the heatexchanger, the water receives additional heat from the second fluid(e.g., fuel processor or oxidizer coolant that is at a highertemperature than the first fluid). The heated water (i.e., heat sink) isthen flowed to its application, in this case a hot water tank.

[0103] Preferably, at least one of the first and second coolant circuitsinclude a radiator having a variable speed radiator fan. The radiatorallows the system to expel heat to ambient when the heat is not neededby the heat sink. In some embodiments, a heat demand sensor is connectedto the controller and adapted to vary a speed of the radiator fan tomaintain a temperature of the heat sink above a predetermined level.

[0104] The system can further include a third heat source and a thirdcoolant circuit, wherein the third coolant circuit is adapted tocirculate a third coolant through the third heat source to transfer heatfrom the second heat source to the heat sink. A third pump is alsoconnected to the controller, which is adapted to vary an output of thethird pump. As an example, the second heat source can be a fuelprocessing reactor, and the third heat source is a system exhaust gasoxidizer, such that heat is transferred from both subsystems to the heatsink.

[0105] In another aspect, the invention provides a method of operating afuel cell system, including the following steps: (1) transferring heatfrom a fuel cell to a first coolant circuit; (2) transferring heat froma second system heat source to a second coolant circuit; (3)transferring heat from each of the first and second coolant circuits toa heat sink; (4) varying a first coolant flow through the first coolantcircuit to maintain a temperature of the fuel cell below a predeterminedlevel; and (5) varying a second coolant flow through the second coolantcircuit to maintain the second system heat source below a predeterminedlevel. Such methods can further comprise selectively flowing at leastone of the first coolant and second coolant through a radiator; andoperating a fan to blow air across the radiator to remove heat from theradiator.

[0106] In another embodiment, the invention provides a fuel cell systemhaving a first heat source (e.g., a fuel cell stack) and a first coolantcircuit, wherein the first coolant circuit is adapted to circulate afirst coolant through the fuel cell stack and remove heat from the firstheat source. A second heat source (e.g., a fuel processing reactor) anda second coolant circuit are also included, wherein the second coolantcircuit is adapted to circulate a second coolant through the second heatsource to remove heat from the second heat source.

[0107] A first heat exchanger in the system includes a first coolantflow path and a second coolant flow path, wherein the heat exchanger isadapted to transfer heat from the first coolant to the second coolantwhen a first temperature of the first coolant is greater than a secondtemperature of the second coolant. A second heat exchanger is locatedalong the second coolant circuit downstream from the first heatexchanger, the second heat exchanger being adapted to transfer heat fromthe second coolant circuit to a heat sink fluid when the second coolantin the second heat exchanger has a higher temperature than the heat sinkfluid. A radiator system is provided that includes a radiator and a fan,the radiator system being located along the second coolant circuitbetween the first heat exchanger and the second heat exchanger. Theradiator is adapted to remove heat from the second coolant circuit.

[0108] A controller is connected to a first pump that is adapted to varya flow of the first coolant. The controller is also connected to asecond pump that is adapted to vary a flow of the second coolant. As anexample, the controller can be configured to vary a speed of the pump orradiator fan to maintain the heat sink fluid above a predeterminedtemperature.

[0109] In another aspect, the invention provides a fuel cell system witha system housing and a heat sink vessel. The heat sink vessel circulatesa heat sink fluid. For example, the heat sink vessel can be a hot watertank that heats water by circulating it within the vessel, where thewater is then flowed to another location for use outside the systemhousing. A portion of the heat sink vessel is contained in an interiorof the system housing. It will be appreciated that in the context ofthis invention, the term portion can mean anywhere from less than 1% to100%. The portion of the heat sink vessel includes a thermallyconductive material. A system component is fixed onto the portion of theheat sink vessel such that heat is transferred from the system componentto the heat sink vessel when a temperature of the system component isgreater than a temperature of the portion of the heat sink vessel. Thesystem component can be any of the following: a pump, a valve, asolenoid, a fuel cell stack end plate, a water tank, a blower, and acircuitry housing.

[0110] In another aspect, a fuel cell system is provided that includes afuel cell, a fuel supply, an oxidant supply, a power demand sensor, aheat demand sensor, and a controller. The fuel cell is adapted toreceive a fuel flow from the fuel supply, and an oxidant flow from theoxidant supply. The controller is connected to each of the fuel supply,oxidant supply, power demand sensor, and heat demand sensor. Thecontroller is further adapted to receive a power demand signal from thepower demand sensor and a heat demand signal from the heat demandsensor.

[0111] In a first state, the controller is configured to reduce at leastone of the fuel flow and oxidant flow when there is no heat demandsignal and no power demand signal. In a second state, the controller isconfigured to increase at least one of the fuel flow and oxidant flowwhen there is no heat demand signal and there is a power demand signal.In a third state, the controller is configured to increase at least oneof the fuel flow and oxidant flow when there is no power demand signaland there is a heat demand signal. In a fourth state, the controller isconfigured to increase at least one of the fuel flow and oxidant flowwhen there is a power demand and a heat demand signal.

[0112] In some embodiments, the power demand sensor is a fuel cellvoltage sensor that produces a power demand signal when a voltage of thefuel cell falls below a predetermined level. The power demand sensor canalso be a fuel cell current sensor that produces a power demand signalwhen an output current of the fuel cell exceeds a predetermined level.The power demand sensor can also include a fuel cell output currentsensor an electrical load sensor, wherein the power demand sensorproduces a power demand signal when an electrical load on the fuel cellexceeds an output current of the fuel cell. It will be appreciated thatthe electrical load on the fuel cell can include a parasitic systemelectrical load and an application electrical load. For example, theparasitic load can refer to internal components such as pumps andblowers that are powered by the fuel cell. The application load canrefer to a residential appliance, as an example.

[0113] The system can further include a coolant circuit and a heat sink,wherein the coolant circuit is adapted to transfer heat from the fuelcell to the heat sink. As an example, the heat demand sensor can be atemperature sensor that produces a heat demand signal when a temperatureof the heat sink is below a predetermined level.

[0114] In one embodiment, the system can include a heat sink, a coolantcircuit, and an oxidizer adapted to oxidize an exhaust gas of the fuelcell. The coolant circuit is configured to transfer heat from the fuelcell to the heat sink, and the heat demand sensor is a temperaturesensor that produces a heat demand signal when a temperature of the heatsink is below a predetermined level. In another embodiment, the coolantcircuit is adapted to transfer heat from the fuel cell to the heat sink,and a radiator is provided to remove heat from the coolant circuit. Theradiator can include a fan connected to the controller, where thecontroller is configured to reduce an output of the fan when there is aheat demand signal. The controller is further configured to increase anoutput of the fan when there is no heat demand signal.

[0115] In another embodiment, the coolant circuit further includes abypass valve and a radiator bypass circuit. The valve is connected tothe controller, and the controller is adapted to actuate the valve todivert a coolant flow from the radiator to the radiator bypass circuitwhen there is a heat demand signal. The controller is further adapted toactuate the valve to divert the coolant flow from the radiator bypasscircuit to the radiator when there is no heat demand signal.

[0116] The system can also include a fuel bypass circuit associated withthe valve. In such a system, the valve is connected to the controller,and the fuel bypass circuit is adapted to divert a portion of the fuelflow from an inlet of the fuel cell to the oxidizer. The controller isconfigured to actuate the valve to divert the portion of fuel flow fromthe fuel cell inlet to the oxidizer when there is a heat demand signal.The controller is further adapted to actuate the valve to divert theportion of fuel flow from the fuel cell inlet to the oxidizer when thereis no heat demand signal. As an example, the controller can include acomputer usable medium (e.g., memory) having computer readable codeembodied thereon (e.g., firmware or software). Preferably, thecontroller is also programmable.

[0117] Embodiments may further include a hydrogen separator, such aselectrochemical hydrogen separator. On this subject, the teachings ofU.S. Pat. No. 6,280,865 are hereby incorporated by reference. Thehydrogen separator is adapted to receive the fuel flow from the fuelprocessor and separate hydrogen from the fuel flow into a reservoir whenthe hydrogen separator is activated. The controller is configured toactivate the hydrogen separator when there is no power demand signal andthere is a heat demand signal.

[0118] As an example, the hydrogen separator can include a membraneelectrode assembly having an anode side and a cathode side. It is wellknown in the art that placing an electric potential across anelectrochemical cell, such as a fuel cell, having no electrical load (asopposed to merely placing an electric load on the fuel cell as in thecase of normal operation) will result in hydrogen beingelectrochemically “pumped” from fuel (e.g., reformate) in the anode tothe cathode. This process proceeds essentially according to the samereactions at the anode and cathode of the fuel cell as in normaloperation. Depending on the mechanical strength of the cell used in sucha process, the hydrogen output is robust enough that such a process canbe used to pressurize a vessel.

[0119] For example, such a cell can be placed along the flow path of thereformate being fed from the fuel processor to the fuel cell. When thereis a heat demand, but no power demand, the controller reacts enough fuelin the fuel cell to produce the desired amount of heat. The excess poweris sunk to the hydrogen separator to pressurize a hydrogen tank (e.g.,at about two atmospheres), which will contain essentially pure hydrogen.The hydrogen tank reservoir can include a valve connected to thecontroller and associated with a conduit to the fuel cell such that thecontroller can selectively open the valve to supply hydrogen to the fuelcell (e.g., in response to a sudden load increase).

[0120] The hydrogen separator can be a PEM fuel cell (e.g., a PEMsandwiched on either side by a platinum based catalyst layer). The anodeside is in fluid connection with the fuel flow from the fuel processor.The anode side and cathode side of the membrane electrode assembly eachhave an electrical connector (e.g., a wire connected to the each of theanode and cathode flow field plates. A power source is connected to theanode and cathode electrical connectors of the membrane electrodeassembly and provides an electric potential across the connectors whenthe separator is in an active state. Similarly, the controller canremove the potential to put the separator in an inactive state. Whilethe separator is in the inactive state, the reformate simply passes byit on the way to the fuel cell without effect. In some embodiments, theseparator can also be used, as can the hydrogen reservoir supply to thefuel cell, when there is a power demand. This mode of operation offersadditional flexibility that can be used by the controller to balancebetween the dynamic behavior of the heat and power demands.

[0121] In another aspect, the invention provides a method of operating afuel cell system including the following steps: (1) providing a fuelflow and an oxidant flow to a fuel cell to produce electricity; (2)providing the electricity to an electrical load; (3) transferring heatfrom the fuel cell to a heat sink by circulating a first coolant througha first coolant circuit, wherein the first coolant circuit is adapted toremove heat from the fuel cell and is further adapted to transfer heatto the heat sink; (4) measuring a thermal parameter of the heat sink;(5) measuring an electrical parameter of the electrical load; (6)measuring a performance parameter of the fuel cell; (7) generating apower demand signal when a power output of the fuel cell indicated bythe performance parameter is less than a power requirement of theelectrical load indicated by the electrical parameter; (8) generating aheat demand signal when the thermal parameter of the heat sink is belowa predetermined level; (9) reducing at least one of the fuel flow andoxidant flow when there is no heat demand signal and no power demandsignal; (10) increasing at least one of the fuel flow and oxidant flowwhen there is no heat demand signal and there is a power demand signal;(11) increasing at least one of the fuel flow and oxidant flow whenthere is no power demand signal and there is a heat demand signal; and(12) increasing at least one of the fuel flow and oxidant flow whenthere is a power demand and a heat demand signal.

[0122] In one embodiment, the method may further include measuring avoltage of the fuel cell; and generating the power demand signal whenthe voltage of the fuel cell falls below a predetermined level. Inanother embodiment, the method can include measuring an output currentof the fuel cell; and generating the power demand signal when the outputcurrent of the fuel cell exceeds a predetermined level. Anotherembodiment can include exhausting fuel gas from the fuel cell to anoxidizer; oxidizing the fuel gas in the oxidizer to generate heat; andtransferring heat from the fuel cell to the heat sink by circulating asecond coolant through a second coolant circuit, wherein the secondcoolant circuit is adapted to remove heat from the oxidizer and isfurther adapted to transfer heat to the heat sink.

[0123] As previously discussed, some embodiments may include a fuelbypass system to allow fuel to be fed directly to the oxidizer when heatis demanded. Methods associated with such embodiments may thus includediverting a portion of the fuel flow from an inlet of the fuel cell tothe oxidizer in response to the heat demand signal.

[0124] In some cases, the first and second coolant circuits can be influid communication, where the first and second coolants are eachportions of a common coolant flow. In other words, the first and secondcoolant circuits are different locations within a single coolantcircuit. In various embodiments, either of the first or second coolantscan be circulated through a radiator to remove heat from the firstcoolant. In one embodiment, the second coolant can be circulated througha radiator to remove heat from the first coolant. For example, systemheat can be absorbed by the first coolant, transferred to the secondcoolant in a heat exchanger, and then expelled to ambient from thesecond coolant in a radiator. One advantage provided by such anarrangement is that the first coolant can be a dielectric stack coolantsuch as deionized water, whereas the second material can be anelectrically conductive material such as tap water. The radiator can bemade from a less expensive material since the tap water is flowedthrough it instead of the deionized water, which might require moreexpensive radiator materials to prevent corrosion.

[0125] As previously discussed, the heat sink can include a water tank.In such cases, the thermal parameter can be a temperature of water inthe water tank. The heat sink can also be a body of air contained in abuilding. In such cases the thermal parameter can be a temperature ofthe air contained in the building. The heat sink can also be a generatorportion of an adsorption cooling system. In such cases, the thermalparameter can be a temperature of the generator portion.

[0126] In another aspect, the invention provides a method of operating afuel cell system including the following steps: (1) providing a fuelflow and an oxidant flow to a fuel cell stack to produce electricity;(2) providing the electricity to an electrical load; (3) transferringheat from the fuel cell stack to a heat sink by circulating a firstcoolant through a first coolant circuit, wherein the first coolantcircuit is adapted to remove heat from the fuel cell stack and isfurther adapted to transfer heat to the heat sink; (4) measuring athermal parameter of the heat sink; (5) generating a heat demand signalwhen the thermal parameter of the heat sink is below a predeterminedlevel; and (6) shorting at least one fuel cell in the fuel cell stack inresponse to the heat demand signal.

[0127] When a fuel cell is electrically shorted (e.g., the anodeelectrode is electrically connected to the cathode electrode,essentially all of the energy produced is converted to heat energy.While this may harm a fuel cell if it is operated is this manner toolong or if the fuel cell is allowed to overheat, operating the cell in ashorted mode for controlled periods of time can provide a desiredincrease in heat from the fuel cell while not harming its performance orreliability. For example, a sulphonated fluorocarbon PEM fuel celloperated in this manner might be kept under 100° C., as an example,either by modulating the coolant flow through the cell or by removingthe short connection at a desired point. As an example, the shortconnection can be a jumper placed between an anode electrode and acathode electrode, wherein the jumper has a switch that can be actuatedby the system controller.

[0128] As in previous examples, the heat sink can be a water tank, abody of air contained in a building, a generator portion of anadsorption cooling system, etc. The thermal parameter generally refersto a temperature of the heat sink.

[0129] In another aspect, the invention provides a method of operating afuel cell system, including the following steps: (1) providing a fuelflow and an oxidant flow to a fuel cell stack to produce electricity;(2) providing the electricity to an electrical load; (3) transferringheat from the fuel cell stack to a heat sink by circulating a firstcoolant through a first coolant circuit, wherein the first coolantcircuit is adapted to remove heat from the fuel cell stack and isfurther adapted to transfer heat to the heat sink; (4) measuring athermal parameter of the heat sink; (5) generating a heat demand signalwhen the thermal parameter of the heat sink is below a predeterminedlevel; and (6) selectively connecting at least two fuel cells in thefuel cell stack in parallel in response to the heat demand signal.

[0130] Without wishing to be bound by theory, a group of fuel cellsgenerally produce a greater amount of waste heat when they are connectedin parallel rather than in series. One reason is that the cellsgenerally operate at a lower efficiency in such a configuration, so thatmore waste heat is generated. Thus, the invention provides an embodimentwhere the balance between the heat and power demand signals isaccommodated by selectively connecting at least two fuel cells within agroup to increase the amount of heat that is generated for a givenamount of power production. Where a system is adapted to selectivelyconnect one or more cells in parallel, the cells that are selectivelyconnected are connected via a switched network, rather than being stackin series as in a conventional stack. For example, two fuel cells may beconnected to a switch that is connected to two electrical paths. Whenthe system controller causes the switch to select one of the paths, thisresults in the cell being connected in series with another cell. Whenthe other path is selected, the cell will be connected in parallel(e.g., connected to a common bus).

[0131] As in previous examples, the heat sink can be a water tank, abody of air contained in a building, a generator portion of anadsorption cooling system, etc. The thermal parameter generally refersto a temperature of the heat sink.

[0132] In another aspect, the invention provides a method of operating afuel cell system, including the following steps: (1) providing a fuelflow and an oxidant flow to a fuel cell stack to produce electricity;(2) providing the electricity to an electrical load; (3) transferringheat from the fuel cell stack to a heat sink by circulating a firstcoolant through a first coolant circuit, wherein the first coolantcircuit is adapted to remove heat from the fuel cell stack and isfurther adapted to transfer heat to the heat sink; (4) measuring athermal parameter of the heat sink; (5) generating a heat demand signalwhen the thermal parameter of the heat sink is below a predeterminedlevel; and (6) selectively operating the fuel cell in a low efficiencymode in response to the heat demand signal.

[0133] There are various fuel cell operating regimes that result inrelatively low efficiency operation and the production of relativelyhigh amounts of waste heat. Prior art systems are generally configuredto avoid such regimes as a means of maximizing system efficiency.However, in systems balancing both heat demand and power demand signals,it may be desirable to switch between such modes. Other examples of lowefficiency operating modes include reactant starvation, operation attemperatures outside the normal operating range of a fuel cell, andproducing a given amount of power at low voltage and high current (e.g.,cell voltages less than 0.4 volts).

[0134] While the invention has been disclosed with respect to a limitednumber of embodiments, those skilled in the art, having the benefit ofthis disclosure, will appreciate numerous modifications and variationstherefrom. It is intended that the invention covers all suchmodifications and variations as fall within the true spirit and scope ofthe invention.

What is claimed is:
 1. A method of operating a fuel cell system,comprising: providing a fuel flow and an oxidant flow to a fuel cellstack to produce electricity; providing the electricity to an electricalload; transferring heat from the fuel cell stack to a heat sink bycirculating a first coolant through a first coolant circuit, wherein thefirst coolant circuit is adapted to remove heat from the fuel cell stackand is further adapted to transfer heat to the heat sink; measuring athermal parameter of the heat sink; generating a heat demand signal whenthe thermal parameter of the heat sink is below a predetermined level;and shorting at least one fuel cell in the fuel cell stack in responseto the heat demand signal.
 2. The method of claim 1, wherein the heatsink comprises a water tank, and wherein the thermal parameter is atemperature of water in the water tank.
 3. The method of claim 2,wherein the heat sink comprises air contained in a building, and whereinthe thermal parameter comprises a temperature of the air contained inthe building.
 4. The method of claim 3, wherein the heat sink comprisesa generator portion of an adsorption cooling system and wherein thethermal parameter is a temperature of the generator portion.
 5. A methodof operating a fuel cell system, comprising: providing a fuel flow andan oxidant flow to a fuel cell stack to produce electricity; providingthe electricity to an electrical load; transferring heat from the fuelcell stack to a heat sink by circulating a first coolant through a firstcoolant circuit, wherein the first coolant circuit is adapted to removeheat from the fuel cell stack and is further adapted to transfer heat tothe heat sink; measuring a thermal parameter of the heat sink;generating a heat demand signal when the thermal parameter of the heatsink is below a predetermined level; and selectively connecting at leasttwo fuel cells in the fuel cell stack in parallel in response to theheat demand signal.
 6. The method of claim 5, wherein the heat sinkcomprises a water tank, and wherein the thermal parameter is atemperature of water in the water tank.
 7. The method of claim 5,wherein the heat sink comprises air contained in a building, and whereinthe thermal parameter comprises a temperature of the air contained inthe building.
 8. The method of claim 5, wherein the heat sink comprisesa generator portion of an adsorption cooling system and wherein thethermal parameter is a temperature of the generator portion.
 9. A methodof operating a fuel cell system, comprising: providing a fuel flow andan oxidant flow to a fuel cell stack to produce electricity; providingthe electricity to an electrical load; transferring heat from the fuelcell stack to a heat sink by circulating a first coolant through a firstcoolant circuit, wherein the first coolant circuit is adapted to removeheat from the fuel cell stack and is further adapted to transfer heat tothe heat sink; measuring a thermal parameter of the heat sink;generating a heat demand signal when the thermal parameter of the heatsink is below a predetermined level; and selectively operating the fuelcell in a low efficiency mode in response to the heat demand signal. 10.The method of claim 9, wherein the low efficiency mode comprises:starving at least one fuel cell in the fuel cell stack of at least oneof the fuel flow and oxidant flow.
 11. The method of claim 9, whereinthe low efficiency mode comprises: increasing an electrical currentflowed from the fuel cell to the electrical load, such that an operatingvoltage of the fuel cell decreases.